1. Field of the Invention
The invention relates to forming protective films on the internal surfaces of a nuclear fuel pin by oxidizing agents. More particularly, the invention relates to renewal of protective films within a nuclear fuel pin by decomposition of material within the pin to provide a source of an oxidizing agent.
2. Description of the Prior Art
Zirconium alloys have been used as clad for nuclear fuel pins which are satisfactory from many standpoints. However, vulnerability of Zircaloy to chemical reaction with iodine, and other elements, inside the fuel pin during operation has limited the performance of the fuel pins. These chemical reactions can produce stress corrosion cracking of the clad and the eventual penetration of the wall of the pin. The result is leaking of the pin. Premature failure of a number of pins in this way can result in a very expensive replacement program and loss of power generation.
If ZrO.sub.2 can be formed and maintained on the inside surface of the cladding, the protection by this coating keeps the zirconium alloy relatively immune to chemical attack from within. Unfortunately, the natural environment inside the fuel pin has an oxidizing potential for only a short time. Reactions with the cladding, and other species present, consume the available oxygen during early operation of the fuel pin.
The oxide surface, formed initially, can be broken by mechanical interaction with the fuel pellets or by other means such as chemical breakdown in the absence of sufficient oxygen. The exposed base-metal, zirconium, cannot be filmed over if there is a lack of sufficient oxidizing potential within the pin. Therefore, the base-metal becomes subject to chemical attach by the fission products available through the material processes operating within the pin. Stress corrosion cracking has often been the result of this attack, resulting in failure by perforation of the cladding.